316 THE REALITIES OF MODERN SCIENCE 



case radiant) is absorbed only in discrete quanta. 

 It is found that if the frequency of the light is main- 

 tained constant the number of electrons emitted in- 

 creases with the intensity of the illumination, but 

 that the speed, and hence the energy, of each electron 

 is independent of this intensity. In other words more 

 quanta are received if the light is more intense, and 

 hence more electrons can be emitted, but each can 

 only absorb the same quantum as before and hence 

 must possess only that amount of energy. On the 

 other hand, if the intensity of the light is kept con- 

 stant and the frequency is increased, the quanta are 

 greater, as is manifested by the increased velocity of 

 the emitted electrons. 



The photoelectric effect, therefore, offers a very 

 convenient method of determining Planck's constant, 

 h, by which the frequency must be multiplied to ob- 

 tain the quantum 1 of energy corresponding to that 

 frequency. The determination involves a knowledge 

 of the charge on the electron. This constant was 

 therefore redetermined by Millikan using his value for 

 e. He found it to be 6.56 X 10~ 27 . 



1 A frequency of about 1.5 XlO 14 is that at which a heated body 

 radiates the maximum energy. This corresponds to a wave length 

 of (3.0X10 10 )/1.5X10 14 , or about two thousandths of a milli- 

 meter, or 2/i as it is abbreviated. The quantum is then (1.5xl0 14 ) 

 (6.56X10- 27 ) or 9.9X10- 13 erg. The lowest frequency visible as 

 red light is 3.75X10 14 , of wavelength X=0.8/x, and its quantum is 

 2.5xlO~ 12 erg. The highest frequency visible as violet light is 

 7.5xl0 14 , that is \=OAn, and the quantum is 5.0xlO~ 12 erg. Ultra- 

 violet light is not transmitted by ordinary glass as well as by 

 quartz. About the highest frequency the latter transmits is 

 1.5xl0 15 , that is X=0.2/x, and the corresponding quantum is 9.9X10" 12 

 erg. 



